The present invention relates to improvements in ballasting for ships, and more particularly, to a ballast-free system for ships.
All ships are designed recognizing Archimedes Principle stating that the weight of a ship is balanced by the weight of the fluid displaced by the watertight hull, termed buoyancy. The approach to ballasting ships has for centuries been the addition of weight to get the ship down to the required ballast drafts forward and aft. Early vessels used solid ballast and then, with the advent of steel ships and mechanical pumps, ships moved to the much more practical water ballast stored in various ballast tanks. The water in these tanks or the residual water and sediment in empty ballast tanks is today the principal culprit for the introduction of nonindigenous aquatic species from one environment to another.
U.S. Pat. No. 6,053,121 assigned to Teekay Shipping Corporation of Nassau, Bahamas sought to reduce the crew effort and pumping power required to accomplish flow-through ballast exchange, by using piping from high pressure at the bow connected to each ballast tank to drive a flow-through ballast exchange process. On the high seas, the conventional ballast tanks are sequentially lowered to a hydrostatic balance level and then connected to the bow high pressure. The bow pressure forces flow through the tank to a low discharge at the forward bottom of the tank. After a period of flow-through, each ballast tank is isolated and pumped back full using the ballast pump.
The present invention includes a completely new approach to the ballasting of ships. By changing the entire thinking about ballasting a ship, a paradigm shift, it may be possible to virtually eliminate the potential for the introduction of nonindigenous aquatic species into the Great Lakes and other coastal waters. Ships must ballast when operating without cargo in order to provide transverse stability, provide bow submergence to prevent slamming structural damage, reduce windage for adequate maneuverability, provide propeller submergence, etc. The current ballast management method of high seas ballast exchange is generally considered to be only partially effective and alternative methods, such as mechanical separation and ultraviolet (UV) light treatment, require significant capital investment, weight, and space.
By making a complete change in thinking, a ship can also achieve its required ballast drafts by changing the volume of the watertight hull in the light (no cargo) condition; i.e., reducing the buoyancy rather than adding weight. During operation, there is a positive hull surface pressure differential between the bow and the stern regions of a ship. The external portion of the hull around the cargo carrying portion of the ship below the desired ballast waterline can be designed to include a group of structural trunks running the full length of the cargo hold. In ballast operations, these trunks can be opened to the sea with an intake opening at the bow and a discharge opening at the stern. These trunks can be flooded, reducing the buoyancy of the hull and allowing the ship to sink to its desired ballast drafts. With the positive pressure drop between the bow and the stern, these trunks can experience a low velocity flow during the entire ballast voyage. This can reduce the watertight volume and buoyancy of the hull in the ballast condition. The ship can then achieve its desired ballast drafts. With flow, the water in these trunks can always be xe2x80x9clocalxe2x80x9d water virtually eliminating the possibility of the introduction of nonindigenous aquatic species into the Great Lakes and other coastal environments. When loading cargo, these trunks can be isolated from the sea by valves and pumped dry using current ballast pumps.
While the present invention appears reasonable and technically feasible, it is believed that ship models will confirm that this change in the overall design of new ships can actually be physically and economically feasible. The goal of the ship models will be to confirm that the ships do not transport ballast or sediments from one point to another and essentially operate ballast-free. It is believed that research using the ship models will confirm and quantify the following key technical and cost issues related to the present invention:
establish the pressure differential and resulting flow rate through the trunks,
establish the effect of the flow diversion through the hull on the resistance and propulsion of the ship using a combination of analyses, Computational Fluid Dynamics (CFD) computations, and model-scale towing tank experiments,
develop a structural design for the ballast-free ship that can provide equivalent structural effectiveness at a comparable cost,
develop details of the inlet and outlet plena, ballast piping, and ballast system controls within an overall ship and engine room arrangement,
develop overall cargo arrangements that can provide the same grain cargo capacity as existing vessels,
verify adequate vessel transverse stability,
analyze ship motions at sea reflecting a higher ballast condition metacentric height GMT and a higher cargo center of gravity,
analyze the damage survival capability relative to the current probabilistic IMO standards to verify at least equivalent safety with the longitudinally subdivided trunks, and
estimate ship construction and operating costs to establish an economic comparison to existing ships with current ballast management options.
Full-scale verification of the present invention is not practical. A true full-scale demonstration would require new ship construction of costly ship modifications. Therefore, large self-propelled, model-scale testing in the University of Michigan Marine Hydrodynamics Laboratory will be used as the most feasible substitute for a full-scale demonstration of the present invention. With success, the present invention can provide a completely new way to design ships so that the risk of the introduction of nonindigenous aquatic species through the ballast water vector might be essentially eliminated.
Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.